1. Field of the Invention
The present invention relates to a toner density detection apparatus, e.g., a toner density detection apparatus of an image forming apparatus which forms a color image.
2. Description of the Related Art
Image forming apparatuses of an electrophotographic type, such as a printer and copying machine, execute density adjustment to stabilize the image density. For example, the image forming apparatuses form a pattern image (toner patch) for density measurement on an image carrier such as a photosensitive body or intermediate transfer member, measure the density of the toner patch, and then feedback-control the laser light amount, process high voltage setting, toner replenishing amount, and the like on the basis of the resultant density signal. Japanese Patent Laid-Open No. 2002-116614 proposes a method of measuring the density of a toner patch.
A conventional method of measuring the density of a toner patch will be described with reference to FIGS. 6 and 7.
Referring to FIG. 6, a light emitting element 1 such as an LED irradiates a toner patch 2 formed on an image carrier 12 such as a photosensitive body or intermediate transfer belt with light. A patch sensor unit 11 includes a polarization beam splitter 3 inserted in the optical path of the light reflected by the toner patch 2, and receiving units 4 and 5 for receiving the s-polarized light (s wave) and p-polarized light (p wave) having passed through the polarization beam splitter 3. The patch sensor unit 11 includes amplifiers 6 and 7. The amplifier 6 amplifies a signal corresponding to the light amount of p-polarized light output from the receiving unit 5 with a predetermined gain, and sends the amplified signal to the subsequent stage. The amplifier 7 amplifies a signal corresponding to the light amount of s-polarized light output from the receiving unit 4 with a predetermined gain, and sends the amplified signal to the subsequent stage. In this manner, the light amounts of two different light components (p-polarized light and s-polarized light) of the light reflected by the toner patch 2 can be independently calculated.
FIG. 7 is a circuit diagram of the receiving units 4 and 5. A variable resistor VR is operated to change a combined resistance (the combined resistance of the resistors VR and R) R′ of the cathode terminal of a receiving element PS and output terminal of the circuit, thus allowing gain adjustment of an operation amplifier OP. The output voltage is expressed by (I×R′+k×Voff), where k is the amplification factor or gain defined by the operation amplifier OP and the variable resistor VR. With this circuit arrangement, the variable resistor VR is adjusted by P and S waves at the time of measurement of a density of the toner patch. This makes it possible to measure the density of a toner patch within a predetermined accuracy.
Referring back to FIG. 6, the signal sent through the amplifier 6 or 7 is input to a controller 10. The amplifier 6 or 7 connects to the input of an A/D converter 8 or 9. The analog signal sent through the amplifier 6 or 7 is converted into digital data by the A/D converter 8 or 9. The controller 10 recognizes detection of the density of the toner patch. In the A/D converters 8 and 9, a Ref voltage 13 is an upper limit.
The prior art disclosed in Japanese Patent Laid-Open No. 2002-116614 executes gain adjustment while the amplifier, which amplifies a signal of an S wave having a narrow dynamic range, has a higher gain than that of the amplifier which amplifies a signal of a P wave having a wide dynamic range.
Since the amplifier is normally formed from an analog circuit, it has low latitude (allowable range) for the environmental characteristic to obtain a sufficient accuracy of amplification factor. The accuracy is deteriorated by a temporal change. As described above, gain adjustment is performed using the variable resistor VR so that the output signal of P or S wave at the time of measurement of the density of the toner that is formed based on a signal with a predetermined density level. Such gain adjustment requires manual operation over a long time and therefore leads to a variation in adjustment.
For these reasons, toner density measurement accuracy sometimes does not have a sufficient accuracy.